Studies on the mechanism of action of the streptococcal cytolytic toxins, streptolysin O (SLO) and streptolysin S, will be continued. The effects of sublethal concentrations of SLO on mammalian cells will be investigated, using pinocytosis and phagocytosis to assess membrane function in macrophages and other cells which have been treated with toxin. We will also continue to study the increased resistance to SLO of mammalian cells in which sterol synthesis has been blocked by hydroxyl derivatives of cholesterol. Gas chromatography will be used to follow changes in membrane sterol content and to relate these changes to decreased susceptibility to SLO. BIBLIOGRAPHIC REFERENCES: Duncan, J. L., and L. Mason. 1976. Characteristics of streptolysin S hemolysis. Infect. Immun. 14: 77-82. Smyth, C. J., and J. L. Duncan. 1977. Thiol-activatable (oxygen-labile) cytolysins. In J. Jeljaszewicz and T. Wadstrom, eds., Bacterial toxins and cell membranes. Academic Press, London, in press.